Oil shale retorting in either above-ground or insitu retorts centers around heating the material, which is typically 85% rock, to about 480.degree. C. (900.degree. F.) or higher in a low oxygen environment. The kerogen and bitumen in the rock are decomposed into oil, gas, and a residual char which remains within the rock. Under favorable circumstances in the laboratory, 70 wt% of the organic matter is converted to oil upon heating to 500.degree. C., and the remainder is converted to about 15% gas and 15% char. These yields change when oil shale is processed in prototype commercial retorts. Oil yield is reduced from 10% to 30% below modified Fischer assay yields, depending on the process, and char and gas yields are correspondingly increased.
The energy content of the organic matter not converted to oil exceeds net process heat requirements for oil shale retorting. The excess depends on the organic content of the shale and the process design, but for well developed processes using a medium grade of shale (roughly 11 to 15 wt% organic matter) the excess can be 100% greater than heat requirements. For instance, the TOSCO II process is based on discarding 100% of the char produced from 11% organic matter shale. The excess can be double or triple this amount when richer material is processed.
The energy contained in the char and gas by-products typically cannot be used efficiently. The gas produced is often greatly diluted with nitrogen and carbon dioxide, and the heat content and its efficiency of usage are greatly reduced. In addition, the energy generated by combustion of the residual char in the rock is considerably less than might be anticipated, because of high temperature endothermic mineral carbonate decompositions which occur when the char is burned. In short, the fuels are in dilute forms and the energy contents have low thermodynamic availability. This severely reduces or eliminates their commercial value. Areas in which western oil shale would be developed contain little or no nearby industrial base which could use such fuel, and the gas and char would require prohibitively costly transport. This combined with very low value causes char and gas in excess of process fuel requirements to be waste disposal problems, rather than saleable by-products.
The above characteristics of retorting are drawbacks, since energy in saleable form is the desired product. Requirements for heating rock to high temperature, excessive conversion of organic matter to char and gas, and the low value of the diluted energy content of these by-products combine to reduce the net saleable energy to 1/3 to 1/2 below that contained in the raw oil shale.
U.S. Pat. No. 3,574,087 to Bergen is an example of an oil shale retorting system in which particles of oil shale fall downwardly through a column of upwardly flowing recycle gas. Solar energy has been utilized in the recovery of hydrocarbon fuels. U.S. Pat. No. 4,290,779 discloses the gasification of coal or other biomass material by passing solar heated recycle gas upwardly through a bed of biomass. The recycle gas is heated by passing downwardly through an external honeycomb jacket surrounding the reactor.
Olsen (U.S. Pat. No. 2,760,920) uses a parabolic concentrator to focus solar rays onto a furnace in a coking process. Russell, Jr., et al. (U.S. Pat. No. 3,868,823) use a solar concentrator to gasify coal or oil. Antal, Jr. (U.S. Pat. No. 3,993,458) discloses a reactor with a quartz window for solar energy conversion of solid waste into a synthetic fuel. Keller (U.S. Pat. No. 4,149,856) focuses solar energy onto coal immersed in water in a solar reactor. Gregg (U.S. Pat. No. 4,229,184) applies focused solar rays onto a vertical moving bed of coal and uses a heliostat mirror to generate steam. Aiman, et al. (U.S. Pat. No. 4,415,339) is another instance of solar coal gasification.
Though these solar heating elements are effective for use in lower temperature chemical processing, they are not believed to be suited for direct heating of solid particles of oil shale to the threshold temperature for pyrolysis, usually at least 750.degree. F. (400.degree. C.).